This invention relates generally to electronic countermeasure systems and more particularly to systems of such type which use digital computers for automatically identifying radio frequency (R.F.) energy sources and assigning countermeasures to such sources when required.
As is known in the art, early electronic countermeasure (ECM) systems received signals from various R.F. energy sources and presented the locations of such sources on a suitable display, such as a cathode ray tube (CRT). An operator would observe the CRT display, tune his receiver to separate in frequency each of the R.F. energy sources, perform manual direction finding, select the appropriate R.F. energy source to be jammed and transmit an appropriate jamming modulation. Other early ECM systems used repeater jammers which transmitted all signals received in a fixed frequency bandpass. A shortcoming of these early ECM systems was that they did not provide automatic R.F. energy source identification and prioritization.
In more modern ECM systems the signal processing R.F. energy source identification and jamming assignment must be done automatically because the R.F. energy source environment, both friendly and hostile, may be so dense that a single operator would not generally be able to perform the R.F. energy source identification task. Such a modern ECM system generally includes receiving equipment, signal processing equipment, a general purpose computer, displays and signal generator and transmission equipment. The receiving equipment and signal processing equipment convert various characteristics of each R.F. energy source into a digital word. These characteristics are typically the R.F. energy source's time of arrival (TOA), angle of arrival (AOA), pulsewidth, amplitude and frequency. These digital words are fed to a general purpose digital computer which provides an appropriate display for the operator and also automatically assigns jamming resources on a somewhat optimum basis to the threatening R.F. energy sources.
In order to establish whether received radio frequency signals are from a valid R.F. energy "emitter" or from "noise," and in order to calculate the pulse repetition interval (PRI) of such signal and thereby establish the characteristics of a valid emitter so that R.F. energy source identification becomes more accurate, it is necessary to sort the received signals so that received signals from the same R.F. energy source are grouped together. The basic idea behind conventional sorting is as follows: For each received radio frequency pulse, the observable parameters (frequency, pulse width, AOA, TOA, etc.) are converted into different portions of a corresponding digital word. Thereafter, when another pulse is received having similar parameters it can be assumed that this pulse came from the same R.F. energy source as the preceding pulse. For example, if a pulse is received at 3.792 GHz and an azimuth of 208.degree. and another pulse 1 millisecond later is received having a frequency 3.793 GHz and an azimuth of 208.degree. it is very likely that the two pulses came from the same R.F. energy source. Unfortunately, during the 1 millisecond interval the receiving apparatus may have received hundreds of pulses from other R.F. energy sources. However, if there were a hundred radars operating at the same time, each with a pulse repetition interval of about 1,000 pulses per second, the environment would comprise 100,000 pulses per second. Thus, on the average there would be 10 microseconds available to sort each one of the incoming pulses into one of a hundred possible R.F. energy sources. If the comparisons were made sequentially, each incoming pulse would have to be compared against 100 R.F. energy sources (or less) whose characteristics were previously observed. In the worst case, the time available for each one of the 100 comparisons is 0.1 microseconds. In view of the present state of the digital computer art such speed requirement means that conventional data processing approaches to data sorting would not be practical.